© 2000 Heron Publishing—Victoria, Canada
Influence of water table decline on growth allocation and endogenous gibberellins in black cottonwood
Stewart B. Rood (1), Karen Zanewich (1), Corey Stefura (1) and John M. Mahoney (2)
1. Department Biological Sciences, University of Lethbridge, Alberta T1K 3M4, Canada / 2. Alberta Environmental Protection, Lethbridge, Alberta T1J 4L1, Canada / Received May 6, 1999
Summary
Cottonwoods occur in riparian areas where water table depth generally varies with the elevation of the adjacent river. Plant
adaptation to the riparian zone requires the coordination of root elongation to maintain contact with the water table during
the summer decline. We investigated the effects of rate of water decline on growth allocation and concentrations of endogenous
gibberellins (GAs) in black cottonwood (Populus trichocarpa Torr. & A. Gray ex Hook.) saplings. Rhizopods were used to achieve water decline rates of 0, 2 and 4 cm day–1. Root elongation approximately doubled in response to the 2 cm day–1 treatment, whereas leaf area was reduced. A water decline rate of 4 cm day–1 led to water stress, as evidenced by reduced growth, increased leaf diffusive resistance, decreased water potential, and
leaf senescence and abscission. Endogenous GAs were extracted, purified and analyzed by gas chromatography–selected ion monitoring
with internal [2H2]GA standards. Across the sampled plant organs, GAs were generally highest in shoot tips and sequentially lower in basal stems,
root tips, leaves and upper roots; GAs were thus abundant in rapidly growing tissues. Of the GAs measured, GA1 tended to predominate, followed sequentially by GA3, GA8, GA19, GA20, GA29 and GA4. There was little relationship between GA concentration and growth allocation across the water table decline treatments,
although GA8 was consistently reduced in plants experiencing water table decline. Because GA8 is the final gibberellin in the metabolic sequence, it might be useful for assessing historic patterns of GAs and growth
rate. This study demonstrated changes in growth allocation in response to water table decline, but provided little evidence
that endogenous GAs play a primary role in the regulation of root elongation in response to water table decline.
Keywords:
Populus trichocarpa, riparian zone, roots, water stress.
